Conventionally, various types of mechanical devices such as indexer devices and robot arms etc. are per se known that drive a driven member to swing such as a table unit or an arm together with a rotating shaft freely rotatably supported on a shaft support member via a bearing member, by linking the driven member to the rotating shaft, and by making the rotating shaft rotate with a rotational drive means such as an electrically operated servo motor or the like.
Now, since an unbalanced torque acts via the driven member on the rotating shaft around its rotational center due to the eccentric load that originates in the dead weight of the driven member itself and in the dead weight of an accessory such as a workpiece etc. that is fixed to the driven member, accordingly it is necessary rotationally to drive the rotating shaft against the resistance of the unbalanced torque. Therefore, in general, in order to alleviate such unbalanced torque acting on the rotating shaft, a balancer mechanism for the rotating shaft including a gas spring or a hydraulic cylinder is often installed to the shaft support member.
For example, in a rotating table device described in Patent Document #1, a construction is disclosed in which one end portion of a crank member is attached to a free end of a rotating shaft that is linked to a table unit, a cylinder body of a balancer cylinder is freely rotatably attached to the other end portion of the crank member, and the tip end portion of a rod that is installed in the cylinder body so as to advance and retract freely is pivotally attached to an installation surface.
In a construction such as that of Patent Document #1 in which a balancer cylinder whose main body is supported by trunnions is installed, there is the problem that the mechanical device becomes undesirably large in size. Accordingly, in the balancer mechanism for a rotating shaft described in Patent Document #2, a construction is disclosed in which a circular cam plate is attached to a free end of a rotating shaft that rotates a table unit, and a cam follower member at the end of an output rod of a gas spring is contacted against the outer circumferential surface of the cam plate. However, with the constructions disclosed in these Patent Documents #1 and #2, there is the problem that entry of foreign substance is quite easy, since the cylinder body and the rotating components are exposed to the exterior.
Furthermore, with a construction such as that of Patent Document #2 in which a cam mechanism is provided at a free end of the rotating shaft, since load is applied via the cam mechanism to the free end of the rotating shaft, accordingly the angle of displacement of the rotating shaft becomes great, and there is a fear that the accuracy of machining of the workpiece and the operational accuracy of the arm may be deteriorated. Accordingly, in the rotating shaft support mechanism disclosed in Patent Document #3, a construction is disclosed in which a cam plate is attached between bearings that support the rotating shaft, a swing arm is provided whose one end portion is linked to a shaft support member and whose other end portion is linked to an end of an output rod of a gas spring, and a cam follower member that is contacted against the outer circumferential surface of the cam plate is attached to the center portion of the swing arm. With this construction of Patent Document #3, since the cylinder body and the swing arm are housed internally to the shaft support member, accordingly there is no ingress of foreign substance, but it is necessary to remove the shaft support member when exchanging the gas spring, so that the working cost increases.